Faith is a bit further along in the forgiveness track than Buffy is. She's been working actively at it, while Buffy has more been trying to maintain status quo with everything. She hasn't had time to think about forgiveness.

Buffy lets out an exasperated sigh. "Faith! Wait," she actually starts after the other slayer. Maybe she should try a little harder, especially since they're in this situation apparently together, whether they want to be or not.